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Fundamental Parameters of Antenna

Parameters of Antenna are:

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Beam AreaBeam widthHalf power beam

width Full Null beam width PolarizationRadiation IntensityBeam Efficiency Antenna field zonesTransmission

formulaDirectivity

Directive GainRadiation ResistanceRadiation efficiencyResolutionAntenna aperture –

physical and effectiveEffective heightTransmission loss as a

function of frequencyAntenna temperature and

signal to noise ratio

Co-ordinate system

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Radiation Pattern

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A mathematical or graphical representation of the radiation properties of antenna such asAmplitude Phase Polarization, etcAs a function of the angular space co-ordinates

is called as radiation pattern.

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FIELD PATTERN: The plot of field either electric |E| or magnetic |H| on a linear scale is called as field pattern.

POWER PATTERN : A plot of the power ( proportional to either |E|2 or magnetic |H|2

) on linear or decibel (dB) scale .

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Directional pattern of Horn antenna

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Omni-directional pattern

Radian & Steradian

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Beam Area

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The beam area or beam solid angle or ΩA of an antenna is given by the integral of the normalized power pattern over sphere (4π sr).

The Beam Area of an antenna can often be describe approximately in terms of the angles subtended by the half-power points of the main lobe in the two principal planes.

Are the HPBW in the tow principal planes, minor lobes being neglected.

Radiation intensity

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The power radiated from an antenna per unit solid angle is called the radiation intensity U (watts/steradian or /square degree).

The normalized power pattern can also be expressed in terms of this parameter as the ration of the radiation intensity as a function of angle, to its maximum value.

Whereas Poynting vector S depends on the distance from the antenna (varying inversely as a square of distance), the radiation intensity U is independent of the distance, assuming in both cases that we are in the far field of the antenna.

Beamwidth

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The angular separation between two identical points on opposite side of pattern maximum is called as beamwidth.

Importance: It is very important figure of merit It is often used as a trade off between it and side lobe

level. As beam width increases side lobe level decreases. & vice versa.

It is also used to describe the resolution capabilities of the antenna to distinguish between two adjacent radiating sources or radar targets.

The most important resolution criterion states that the “Resolution capability of an antenna to distinguish between two sources is equal to half of the first-null beamwidth”.

2 sources separated by angular distance ≥ of an antenna with a uniform distribution can be resolved.

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Half Power Beamwidth (HPBW): In a plane containing the direction of the

maximum beam, the angle between the two directions in which the radiation intensity is ½ value of beam is called as HPBW.

First Null Beamwidth (FNBW):The angular separation between the first

nulls of the patterns is referred to as the FNBW.

Beamwidth

Polarization

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Polarization is the direction of the electric field and is the same as the physical attitude of the antennaA vertical antenna will transmit a vertically polarized

waveThe receive and transmit antennas need to possess the

same polarization

Beam Efficiency

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The (total) beam area ΩA (or beam solid angle) consists of the main beam area (or solid angle) ΩM plus the minor lobe area Ωm.

The ratio of the main beam area to the (total) beam area is called the (main) beam efficiency.

The ratio of the minor lobe area to the (total) beam area is called the Stray Factor.

It follows that

Directivity

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The directivity of an antenna is equal to the ratio of the maximum power density to its average value over a sphere as observed in the far field of an antenna.

Directivity from pattern :

The directivity is also the ratio of the area of a sphere (4π sr) to the beam area ΩA of the antenna.

Directivity from beam area:

The smaller the beam area, the larger the directivity D.

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For an antenna that radiates over only half a sphere the beam area ΩA = 2π sr,

dBi = decibels over isotropic.

For, Ideal isotropic antenna

HEMISPHERE

ISOTROPIC

Radiation pattern of dipole λ=0.5

GAIN

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Used as a figure of merit.The ability of an antenna or antenna system to

concentrate the radiated power in a given direction or conversely to absorb effectively the incident power from that direction is specified by various antenna terms i.e. antenna gain or simply gain or directive gain or power gain or directivity.

Definition of GAIN is:Gain of antenna without involving the antenna

efficiency is defined as:

Reference antenna may be an isotropic antenna or lossless antenna.

Often gain of an antenna is expressed in decibel ratio i.e.

Directive Gain

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The extent to which a practical antenna concentrates its radiated energy relative to that of some standard antenna is termed as directive gain.

Directive gain is the ratio of the radiation intensity in that direction to the average radiated power.

Directive Gain solely depends on the distribution of radiated power in space. It does not depend upon the power input to the antenna, antenna losses or the power consumed in a terminating resistance.

Comparison between Directivity and Gain

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The ratio of the gain to the directivity is the antenna efficiency factor.

G=kD.k = efficiency factor (0 ≤ k ≤ 1).

Dimensionless.If an antenna has not any losses like ohmic,

dielectric mismatch i.e. 100% efficient, then directivity and gain are same.

For an antenna with losses, gain will be less than directivity by factor which corresponds to efficiency.

Resolution

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Resolution of antenna may be defined as equal to the half the beam width between first null (FNBW)/2.

When the antenna beam maximum is aligned with one satellite, the first null coincides with the adjacent satellite.

Half the beam width between first nulls is approximately equal to the half-power beam width,

Effective height

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Effective height may be defined as the ratio of the induced voltage to the incident field

Represents the effectiveness of an antenna as radiator or collector of electromagnetic wave energy.

It indicates how for an antenna is effective in transmitting or receiving the electromagnetic wave energy.

lh

E

Radiation Resistance

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The radiation resistance (Rr) is thus defined as that fictitious resistance which, when substituted in series with the antenna will consume the same power as is actually radiated.

The radiation resistance represents, Total energy radiating form transmitting antenna Current flowing in the antenna

The value of radiation resistance depends on Configuration of antenna The point where radiation resistance is considered Location of antenna w.r.t. grounds and other objects,

and Ratio of length of diameter of the conductor used. Corona discharge – a luminous discharge round the

surface of antenna due to ionization of air etc.